The operation of chain type conveyor drives is frequently characterized by irregular chain movement called chordal action. Chordal action manifests itself in the form of cyclical velocity variations of the conveyor. In conventional conveyor drives this cyclical velocity variation can be as much as 5-8% of the average velocity of the conveyor. While such variation may be tolerated in certain manufacturing processes where a smooth, continuous drive is not required; i.e., crusher feed conveyors at a quarry, more sophisticated processes; i.e., lamination processes, require the steady, even movement or progress of the manufactured good. Any variation in the speed of the conveyor, even comparatively minor, can seriously effect the integral quality of the finished product.
Of course not all types of conveyors suffer from this problem. Endless belt conveyors, which are driven by a smooth drum at either end of the curved turn arounds, do not have the requisite physical characteristics which allow chordal action to occur unless, of course, the driving drum is itself driven by a chain drive system that creates chordal action. These belt type conveyors, however, have limited use in some manufacturing processes and other problems are associated with their drive systems.
Conventional chain conveyor drive systems, on the other hand, use some form of toothed drive mechanism which engages the conveyor. The toothed drive can be in the form of either a large sprocket, usually at one end of the conveyor, which directly engages a drive chain attached to the moving portion of the conveyor, or a cat drive system which engages the conveyor at some midpoint location between the ends. In both systems, the former perhaps more than the later, chordal action is present. In the large sprocket system the chordal action effect is easily described as follows: Each successive link pin of the conveyor drive chain engages the drive sprocket at a radius point below the point of tangency between the conveyor drive sprocket and the conveyor drive chain. The velocity of the drive chain therefore increases as the link pin is pulled or raised upwardly to the point of tangency. This occurs because the perpendicular vector component of the velocity imparted by the drive sprocket to the link pin goes to zero while the vector component of velocity parallel to the direction of conveyor travel is maximized. This cyclical increase in the parallel or horizontal vector will cause a jump in conveyor movement.
Use of a cat drive system for conveyors will reduce but not eliminate chordal action. In such a cat drive, protruding teeth are mounted on a smaller chain, directly driven through reduction gearing by an electric motor, with those teeth engaging the conveyor drive pins, which protrude from the under side of the conveyor, so that the conveyor is moved along. At the point of disengagement, the tooth on the smaller chain is pulled away from the conveyor drive pins as it wraps around its own drive sprocket.
Since the cat drive is itself a miniature chain drive system, chordal action is inherent in its operation. Although chordal action is reduced, as compared to a large sprocket drive, it is nonetheless present and uneven motion is imparted thereby to the driven conveyor. Again, a steady, even, conveyor velocity is not achieved.
Many methods of eliminating chordal action in chain drives have been tried: Larger drive sprockets with greater numbers of smaller dimensioned teeth; offset drive sprockets which halve the action, for an otherwise equal amount of driving teeth; scrupulously machined sprockets with precision alignment, etc. None of these methods, however, has succeeded.
The present invention eliminates the cyclical velocity variations of chain drives by ridding the drive system of the physical characteristics of conventional chain drives which cause the chordal action.